WASHINGTON – Insurers have raised concerns that too few young people are signing up for heath insurance through the ObamaCare exchanges after newly released statistics showed that less than a quarter of people who have enrolled are between the ages of 18 and 34.

According to the numbers released Monday by the U.S. Department of Health and Human Services, only 24 percent – or 489,460 – of the 2.2 million people who signed up for ACA were in the coveted 18-to-34 age range. That means the government has hit only 18 percent of its stated goal of registering 2.7 million adults in the 18-to-34 age range.

Experts have predicted that the program will need roughly 40 percent of enrollees to be in that prime demographic in order to be fiscally solvent. Adults ages 55 to 64 made up 33 percent of the total number of Americans who signed up, the largest group represented in the data.

ObamaCare needs so-called 'invincibles' -- healthy young adults -- to sign up in coming months to help offset the costs of older and less healthy enrollees. If that doesn’t happen, insurers could be forced to raise the rates, making the costs and future of ObamaCare uncertain.

"This is concerning to us that we're seeing this portion come in so old," Marty Anderson, marketing director for Wisconsin-based Security Health Plan, told the Wall Street Journal.

Allan Einboden, chief executive of Scott & White Health Plan, told the Journal that the data was "more negative than we thought it was going to be," and added that several requests to pre-authorize surgeries for enrollees in the first days of their coverage had executives worried about costs.

Robert Laszewski, a consultant who works with insurers, told The New York Times that "You need healthy people of all ages ... the program is not ramping up fast enough to guarantee a good balance of healthy and sick people, which you need to sustain the program."

Administration officials, though, were upbeat in describing the numbers Monday. They said they're in a "solid place," and noted that this age group makes up just 26 percent of the general population.

Monday’s report was the first time the government has released demographic data on the performance of President Obama’s signature health care overhaul. In all, about 1.8 million people enrolled in new individual health plans through the law in December, bringing the total number of new enrollments between Oct. 1 and Dec.28 to about 2.2 million.

"There's no way to spin it: youth enrollment has been a bust so far," Brendan Buck, a spokesman for House Speaker John Boehner, said in a statement following the release. "When they see that ObamaCare offers high costs for limited access to doctors – if the enrollment goes through at all – it's no surprise that young people aren't rushing to sign up."

As analysts have told Fox News, insurance companies have said they may want to take advantage of the "risk pool" provisions -- which involve the federal government covering 80 percent of any losses associated with an older and unhealthier base of customers.

According to HHS, 79 percent of ObamaCare customers selected a plan with financial assistance. While only 2.2 million people have actually signed up, 44.5 million called or visited state and federal websites.

Of the 2.2 million who have signed up, 54 percent are female and 46 percent are male, HHS said.

Individuals and families are able to choose from four plans offered through the federal marketplace -- bronze, silver, gold and platinum.

Sixty percent selected a silver plan, while 20 percent selected a bronze plan, according to HHS.